Freeze drying method and apparatus

ABSTRACT

Freeze drying methods and apparatus in which a product is frozen onto a conveyor in a vacuum vessel compartment in which the pressure is high enough to keep volatiles from evolving from the product. The product is then conveyed through a drying compartment maintained at a lower pressure where heat is applied to the product to evolve moisture from it. Thereafter, the dried product is removed from the conveyor and the vacuum vessel.

United States Patent FREEZER BRINE COOLER Oppenheimer 34/5 LIQUID HEATER Smith, Jr. 51 June 26, 1973 [5 FREEZE DRYING METHOD AND 3,218,731 11/1965 Stinchfield 34/242 x APPARATUS 2,515,098 7/1950 Smith, Jr 34/5 2,471,325 5/1949 Hickman 34/5 Inventor: Home m h, J Rlchmond, 2,460,197 1/1949 Smith, Jr 34/5 x [73] Assignee: Smitherm Industries, Inc.,

Richmond, Va. Primary Examiner--William F. O'Dea Assisgqnt E5qry iner-William C. Anderson 7 [22] Ffled' July 1972 Attorney-William A. Strauch, Richard D. Multer, [21] Appl. No.: 276,556 James E. Nolan et al.

' 57 ABSTRACT [52] US. Cl 34/5, 34/92, 34/15 1 511 1m. (:1. F26b 5/06 Freeze drying methods and apparatus in which a p [58] Field 61 Search 34/5, 15, 92 w is frozen onto a conveyor in a vawum vessel partment in which the pressure is high enough to keep 5 References Cited volatiles from evolving from the product. The product UNITED STATES PATENTS is then conveyed through a drying compartment main- 3 62 7 tained at a lower pressure where heat is applied to the gg ga product to evolve moisture from it. Thereafter, the 3 362 835 1/1968 T116562 a] l i I i U 34/5 X dried product is removed from the conveyor and the 3,323,225 6/1967 Achucarro 34/5 x vacuum vessel- 3,233,333 2/1966' 9 Claims, 7 Drawing Figures DILUTE COLD BRlNE BRINE COOLER ZOO RE BOlLER HOT BRlNE June 26, 1973 United States Patent [191 Smi mmm t. M. 4 m I u i, O mwwm .nm IQ x ./T .r \w A M W m 4 H 0 1 m 8 m w u A] k Y m m u M. 2 m z w a m I? 1 J w Mu a z w w B )Q E u m m M u M x 6 FREEZE DRYING METHOD AND APPARATUS This invention relates to freeze drying and, more particularly, to novel, improved methods and apparatus for carrying out freeze drying on a continuuos basis.

Continuous type freeze dryers are disclosed in my prior U.S. Pat. Nos. 2,515,098 issued July I1, 1959 and 3,226,169 issued Aug. 6, 1966. In the apparatus disclosed in these patents the material to be dried is frozen, introduced into a vacuum chamber, and deposited on one end of an endless belt conveyor. As the material moves through the vacuum chamber on the conveyor, moisture is evolved from it by radiant heat emitted from one or more radiators adjacent the material bearing leg of the conveyor. At the end of the vacuum chamber opposite that where it is introduced, the dried material is removed from the conveyor belt and then from the vacuum chamber.

One of the problems with continuous freeze dryers is that of keeping the product on the conveyor. The product cannot be introduced into the vacuum vessel of typical freeze drying apparatus in liquid form because volatiles in the product will flash into vapor at the low pressures which exist in the vessel, often resulting in a marked decrease in the quality of the product.

Accordingly, in the typical installation, the product is first converted into a frozen powder which is introduced into the vacuum vessel and spread onto the conveyor. Considerable difficulty may be experienced with the loose particles bouncing or flying off the conveyor when the product is introduced into the vacuum vessel in powdered form. This can result in a build-up of powdered product within the vacuum vessel great enough to materially reduce the efficiency and/or prevent proper operation of the apparatus.

Periodic cleaning of the vacuum vessel to remove the build-up of the product is impractical as is frequent shutting down of the apparatus to remove the powdered product. Accordingly, the tendency of the product to leave the conveyor is a serious problem.

Another disadvantage of spreading the powder on the conveyor in powdered form is that the layer of product must be relatively thick for the apparatus to handle an acceptable volume of material. This makes it diffucult for radiant energy to penetrate to the center of the product layer. Also, a layer of powdered material has low thermal conductivity. Because of these properties of the layer, the removal of the moisture from the product cannot be accomplished with high efficiency.

In my U.S. Pat. Nos. 3,324,565 issued June 13, 1967, and 3,621,587 issued Nov. 23, 1971, I pointed out that the problem just discussed could be solved by bonding frozen particles of the product to each other and to the conveyor after the product was spread on the conveyor to form a thin, more-or-less continuous layer of product adhered to the conveyor. Specifically, I disclosed that, by passing the conveyor and the product on it through the nip between two rolls, advantage could be taken of the phenomenon of relegation to produce a layer of product of the character just described.

As pointed out in the earlier of the patents identified in the preceding paragraph, the formation of the product into a thin, coherent layer solved the problem of loose particles flying off the conveyor and building up inside the vacuum vessel. This also increased the drying efficiency as the layer was both reduced in thickness and made more conductive.

I have now discovered an even more effective technique for forming a frozen layer of product on a freeze dryer conveyor. Accordingly, by employing the present invention, products can be more efficiently and economically dried than has heretofore been possible.

In the present invention the product is introduced into what I term a freezing compartment in an evacuated vacuum vessel and is formed into a layer on an endless belt or other conveyor. The conveyor is then cooled or chilled to freeze the product to it.

The pressure in the freezing compartment is maintained above the vapor pressure of the product so that volatiles will not evolve from it as it is being spread on and frozen to the conveyor. This also keeps the product from turning to icy or snowlike particles while it is being introduced into and spread on the conveyor.

From the freezing compartment the frozen product is conducted by the conveyor into and through a drying compartment where the pressure is maintained at a level much lower than that in the freezing compartment; viz., below the triple point of water. In this compartment the moisture is sublimed from the product by applying radiant heat to it as described in my already issued patents. Then the dried product is removed from the drying compartment and the vacuum vessel.

It is the primary object of the present invention to provide novel, improved freeze drying methods and apparatus of the continuous type.

Other important but more specific objects of the invention reside in the provision of freeze drying methods and apparatus in accord with the preceding object:

I. by which the freeze drying of liquid and semiliquid products can be economically and efficiently accomplished.

2. which are relatively simple and which can be provided and employed with only relatively modest capital expenditures.

3. which simplify the freeze drying of liquid and semiliquid products by permitting the product to be introduced into the vacuum vessel in liquid form.

4. which include various combinations of the foregoing and other desirable attributes.

Other important objects and features and additional advantages of the invention will become apparent from the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing, in which:

FIG. I -is a schematic illustration of a freeze drying system constructed in accord with the principles of the present invention with certain components including a vapor producing system eliminated for the sake of clary;

FIG. 2 is a generally diagrammatic longitudinal section through a vacuum vessel employed in the system of FIG. 1;

FIG. 3 is a section through the vacuum vessel taken substantially along line 33 of FIG. 2;

FIG..4 is a fragment of FIG. 3 to an enlarged scale, showing in more detail an arrangement for distributing a product to be dried on an endless conveyor housed in the vacuum vessel;

FIG. 5 is a section through part of the product distributing arrangement, taken substantially along line 5-5 of FIG. 4;

FIG. 6 is a section through a cooled roll-located at the discharge end of a drying compartment in the vacuum vessel; and

FIG. 7 is a section showing an arrangement for freezing the product onto the conveyor in a freezing compartment in the vacuum vessel and is taken substantially along line 77 of FIG. 2.

In the novel method of freeze drying disclosed herein, a liquid or semiliquid product is frozen in a thin, uniform layer on a conveyor in an environment in which the pressure is high enough to keep volatiles from evolving from the product. The frozen layer of product is then conveyed into a drying chamber where the pressure is below the triple point of water. Here, radiant heat is applied to the frozen product to sublime the moisture from it. Finally, the dried product is cooled, removed from the conveyor, and discharged from the drying chamber.

I Referring now to the drawing, FIG. 1 illustrates freeze drying apparatus 20 including a novel vacuum vessel 22 in which the foregoing steps may be carried out.

As shown in more detail in FIG. 2, vacuum vessel 22 includes a shell 24 divided by an insulated, vertical partition 26 into a freezing compartment 28 and a drying compartment 30.

Housed in the vacuum vessel in vertically spaced apart relationship are a plurality of endless belt type conveyors. Three such conveyors, 32, 34, and 36 are shown in FIG. 2. This number may, however, be varied, depending upon the application of the invention.

Each of the three endless conveyors includes an endless belt 38 which has material bearing and return legs 40 and 42 and is trained around rolls 44 and 46 at opposite ends of the conveyor. One or both of these rolls is driven by an appropriate drive arrangement (not shown) to effect movement of the material bearing leg through the vacuum vessel in the direction shown by arrow 48.

The material bearing and return legs 40 and 42 of each conveyor extend through apertures 50 and 52 in partition 26. Flexible seals 54 are disposed in each aperture 52 above and below the return leg 42 of the conveyor belt 38 to reduce the flow of gases between the freezing and drying compartments 28 and 30.

Referring now to FIGS. 2-5, the product to be dried is introduced into vacuum vessel 22 in liquid or semiliq uid form from a receptacle 56 through a main conduit 58. A valve 60 in the main conduit controls the rate at which the product flows into the vacuum vessel.

From the main conduit, the product flows into one of three longitudinally extending branch conduits 61, 62, and 64. As best shown in FIG. 2, one of these conduits is located above the material bearing leg 40 of each of the three conveyors 32, 34, and 36. Valves 66, 68, and 70 in the branch conduits proportion the product among the three branch conduits.

Referring now specifically to FIG. 3, transversely extending headers 72, 74, and 76, which span the material bearing legs 40 of the three conveyors, are connected to the ends of conduits 61, 62, and'64 opposite main conduit 58. Formed in the lower sides of the headers are discharge holes or apertures 78 (see' FIGS. 4 and The product flows from the branch conduits into the headers and then through these apertures onto the material bearing legs 40 of the conveyor belts to form layers 80 of the product thereon.

Typically, layers 80 will be from one thiry-second to one-sixteenth of an inch thick. In any event, however, the number of apertures 78, their size, and the spacing between them as well as the rate of flow of the product onto the belt and the rate of movement of the latter will be coordinated to produce a layerwhich has the selected thickness and is uniformly thick across the conveyorbelt.

While the just described arrangement is satisfacgory for distrubuting the product to be dried on the conveyor belts, it is by no means essential that it be used. Curtain coaters, T-type and coat hanger dies, and other of the numerous devices availabe for spreading flowable materials on flexible webs can be employed instead, if desired.

The next step in my novel process is to freeze the product to the material bearing leg 40 of the conveyor belt on which it is spread.

There are several ways in which this can be accomplished. The exemplary, illustrated arrangement (see FIGS. 1, 2, 3, and 7) includes three reservoirs 82 having side walls 84 and end walls 86 and an open upper end. One such reservoir is located beneath and immediately adjacent the material bearing leg 40 of each of the three conveyors 32, 34, and 36.

An appropriate refrigerant such as a cryohydrate, lithium chloride brine or solution is chilled in a refrigeration unit 88. Such units are commercially available and will, accordingly, not be described in detail herein.

From the refrigeration unit, the cold refrigerant flows through a supply line 90 into a transversely extending header 92 located in the bottom of each reservoir 82. As best shown in FIG. 7, the refrigerant then flows out of the open ends of the header and circulates into contact with the lower side of the conveyor belt material bearing leg 40, reducing its temperature and causing the product to freeze and adhere to the belt. The refrigerant then spills over the side walls 84 of the reservoir into traps 94. From these, the refrigerant is circulated through return conduits 96 (only one of which is shown in FIG. 1) to the refrigeration unit 88, where it is recooled and recirculated.

As in the case of the product supplying and layer forming arrangement, that employed to freeze the proudct to the conveyors need not be of the particular construction illustrated in the drawing. For example, it may be replaced with spray nozzles for projecting the refrigerant against the undersides of the material bearing legs of the conveyor belts. Also, doctor blades, squeeges, etc. may be located adjacent partition 26 to remove from the undersides of the belts any refrigerant which may adhere to them.

It was previously pointed out that it is important to keep the pressure in freezing compartment 28 high enough that volatiles will not evolve from the product while it is being spread on and frozen to conveyors 32, 34, and 36. If this is not done, aroma and flavor producing'and/or other desirable constituents can be lost from the product, resulting in an undesired deterioration in its quality.

Furthermore, if the requisite minimum pressure is not maintained in the freezing compartment, the product will be instantly reduced to particles as it flows out of the distribution headers. As discussed above, these particles will not stick to the conveyors but will accumulate on the inner surfaces of shell 24, on partition 26, and on other components in the freezing compartment, reducing the efficiency of the operation or stopping it altogether.

The pressure is preferably kept at the desired level in freezing compartment 28 by maintinaing an atmosphere of water vapor in it. One arrangement for doing this, shown in FIG. 2, includes a water reservoir 98 hav ing an open top. The reservoir is located'in the lower reaches of the freezing compartment. Water is supplied to the reservoir through line 100 and a float type control 102 of conventional construction, keeping water at a selected level in the reservoir.

Water is evaporated from the body of liquid in the reservoir at a controlled rate to maintain the pressure in the freezing compartment constant by a conventional resistance heater 104 in the reservoir.

The pressure maintained in the freezing compartment will vary, depending primarily upon' the product being dried, although it will always be well below atmospheric pressure. In the freeze drying of coffee, for example, the pressure in this compartment may be maintained at a level on the order of 5 plus mm of mercury absolute. i

At 5.3 mm Hg absolute, the temperature in the freezing compartment will be 36F. This is highenough to keep the product from freezing as coffee extracts typically freeze at 27-27.5F. At the same time, the pressure will be high enough to keep voltatiles from evolving, but not so high as to produce an unacceptable pressure differential between and flow of vapor from compartment 28 into compartment 30.

The particular arrangement employed to maintain the vapor pressure at the selected level in freezing compartment 28 is not critical. Alternates to the illustrated arrangement include steam jets, thermocompressor systems for compressing vapor in drying compartment 30 and returning it to the freezing compartment, etc.

Referring again to FIG. 2,the product frozen to the material bearing legs 40 of the three conveyors is transported by the conveyors through partition 26 into the drying compartment 30 of the vacuum vessel. This compartment is maintained at a pressure below 4.6 mm of mercury absolute and typically at about 1.5 mm mercury.

Radiant heaters or radiators 106 and 108 are disposed on opposite sides of each material bearing conveyor leg 40 in drying compartment 30 to apply radiant heat to the product.

The conveyor belts can be made of metals such as stainless steel or bronze or of synthetic materials such as Mylar or the like. The upper sides of the product layers will be heated by direct radiation from heaters 106. If the belts are made of metal, radiators 108 will heat the undersides of the material bearing legs by radiation, and heat will be conveyed through the latter to the product layers by conduction. 1f the belts are made of Mylar or the like, the radiant energy from heaters 108 will pass through the belts 'to the undersides of the product layers.

At the lower pressure mentioned above, water in the product will pass directly from the frozen state to the vapor state at temperatures of F. or higher. Radiators 106 and 108 supply the heat necessary to raise the temperature of the product above this level.

The radiators may be of any desired type such as, for example, those disclosed in my U.S. Pat. Nos. 3,262,494 issued July 26, 1966; 3,285,514 issued Nov. 15, 1966; and 3,305,011 issued Feb. 21, 1967. The radiators are heated by circulating a fluid, preferably liquid, heat transfer medium through them. Suitable heat transfer liquids are disclosed in my U.S. Pat. No. 3,236,292 issued Feb. 22, 1966.

The system for heating and circulating the heat transfer medium will typically include a conventional liquid heater (see FIG. 1) connected to the radiators by a supply conduit arrangement indicated generally by reference character 112. The heat transfer medium is returned to the heater by a conduit system similarly identified by reference character 114.

The dried product is cooled to harden it, separated from the conveyor, and discharged from the vacuum vessel. The product may be cooled by chilling the rolls 46 at the discharge ends of conveyors 32, 34, and 36 so that heat will be conducted away from the product as the material bearing legs 40 of the conveyor belts pass around the rolls.

As shown in FIG. 6, the rolls 46 employed in the exemplary system 20 each have a cylindrical shell 122 supported from a central shaft 124 as by end members 126. The shaft 124 is rotatably supported in bearings 128 attached to structrual components in the vacuum vessel which are identified generally by reference character 130.

Disposed within each roll in heat transfer relationship to its shell 122 is a spiral coil 132, which extends substantially from end-to-end of the roll.

A refrigerant issupplied from a refrigeration unit (which is not shown but may be of the same type as unit 88) through a supply line 134 and a rotary coupling 136 to a passage 138 in the shaft 124 of each roll. From this passage, the refrigerant flows through coil 132, a passage 140 in the opposite end of the shaft, a rotary coupling 142, and a return line 144 back to the refrigeration unit.

Other types of rolls through which a heat transfer,

and 162. A pivoted gate 164 can be swung from the position shown in full lines to that shown in dotted lines to direct the dried product from conduit 158 into a selected one of the two hoppers.

From the hopper the product flows through a rota pocket valve 166 or 168 or other device capable of isolating the vacuum vessel from the surrounding environment into one of two transfer conduits 170 and 172 which can be evacuated through vacuum lines 174 and 176. From the transfer conduit, the product flows through a second pocket valve 178 or 180 into a conduit 182 or 184 for transfer to a packaging or other processing station.

It is necessary to continuously remove from the vacuum vessel. the water vapor evaporated from the product in drying chamber 30 as well as the water vapor which flows into the drying chamber from freezing chamber 28 through partition 26 because of the pressure differential between the two chambers. The system employed in the apparatus 20 of FIG. 1 for this purpose includes a condensor 186 connected to vacuum vessel 22 by a conduit 188 and to a vacuum pump 190 by a conduit 192.

Vacuum pump 188, which is also used to pump down vacuum vessel 22 when the system is started up, draws the accumulated water vapor from vacuum vessel 22 through conduit 188 and upwardly through condensor 186. Any noncondensible gases mixed with the water vapor are discharged from the system by the pump through conduits 192 and 193.

A cold sorbent such as lithium chloride brine is pumped into the upper end of condensor 186 through a conduit 194 and flows downwardly through it, absorbinb and condensing the vapor moving upwardly through the condensor as it does so.

The sorbent is diluted by the condensed water vapor and heated by the latent heat given up as the water vapor condenses. Accordingly, it is necessary to reconcentrate and cool the lithium chloride solution after it leaves the condensor.

The heated, dilute sorbent is therefore recirculated from condensor 186 through return conduit 196 to a brine cooler 198 to reduce its temperature before it is returned through conduit 194 to the condensor.

To keep the sorbent at the desired concentration, a portion of the diluted sorbent discharged from cndensor 186 is diverted from conduit 196 through conduit 200, a coil 202 in heat exchanger 204, and conduit 206 to a sorbent concentrator or reboiler 207. Here, excess water is boiled off the sorbent. This vapor may be conducted to the freezing compartment 28 in vacuum vessel 22 to provide at least part of the needed water vapor therein, if desired.) The heat is supplied by circulating a fluid heat transfer medium through the concentrator from liquid heater 110 by way of conduits 208 and 210.

The concentrated sorbent flows into conduit 211 and through a coil 212 in heat exchanger 204 disposed in heat transfer relationship to coil 202. The sorbent in coil 212 gives up heat to the dilute sorbent flowing through coil 202, reducing the temperature of the reconcentrated sorbent and adding sensible heat to the dilute sorbent flowing to sorbent concentrator 207.

From the heat exchanger the sorbent flows through conduit 214 to brine cooler 198, where it is combined with the sorbent circulated directly from condensor 186 to the cooler.

The foregoing system isdescribed in more detail in my US. Pat. No. 3,621,587 issued Nov. 23, 1971, to which the reader may refer if desired.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is: i I

1.. Freeze drying apparatus comprising: a vacuum vessel; means dividing said vacuum vessel into a freezing compartment and a drying compartment; means for evacuating said vessel; means for conveying a product to be dried from said freezing compartment into and through said drying compartment; means for introducing a liquid or 'semiliquid product to be dried into the freezing compartment and onto the conveying means in a layer, means for cooling said conveying means to freeze the product thereto; means for keeping the pressure in the freezing compartment at a selected level which is sufficiently high to prevent volatiles from evolving from'the product until it is frozen to the conveying means and to prevent the product from freezing until it is on the conveying means; means for heating the product as it is conveyed through the drying compartment to evolve water therefrom; and means for removing the dried product from the vessel.

2. The freeze drying apparatus of claim 1, wherein the means for providing water vapor in said freezing compartment comprises a receptacle in said freezing compartment, means for supplying water to said receptacle, and means for maintaining the water in said receptacle at a temperature sufficiently high that said water will volatilize at the pressure in the freezing compartment.

3. The freeze drying appartus of claim 1, wherein the means for cooling the product conveying means comprises means for effecting the flow of a refrigerant into heat exchange relationship with said conveyor means.

4. Freeze drying apparatus according to claim 1, wherein the product conveying means has a material bearing leg and wherein the means for cooling said con veying means comprises a reservoir beneath said material bearing leg and means for effecting a flow of said coolant into said reservoir and out the upper end thereof and into heat transfer relationship with said material bearing leg.

5. The freeze drying apparatus of claim 1, wherein the means for maintaining the pressure in the freezing compartment at the selected level is means for maintainingan atmosphere of water vapor in said freezing compartment.

6. The freeze drying apparatus of claim 5, wherein there is a partition in said vacuum vessel dividing it into said freezing and drying compartments, there being apertures in said partition through which said conveyor can pass and said apparatus further including means for removing from said vacuum vessel moisture evolved from the product in the drying compartment and water vapor flowing from the freezing compartment into the drying compartment through the apertures in the partition.

7. The method offreeze drying a product, comprising the steps of: introducing the product to be dried into a zone maintained at a pressure which is below atmospheric but is sufficiently high to keep volatile constituents of said product from vaporizing; spreading the product into a layer; freezing the product in said layer; reducing the pressure on the frozen product to a pressure below 4.6 mm of mercury; and heating the product to effect the sublimation of water from the product and thereby reduce its moisture content.

8. The method of claim 7, wherein said product is spread into a .layer which is not more than onesixteenth inch thick.

9. The method of claim 7, wherein the selected pres-- atmosphere of evaporated water.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 34740.860 Dated J1me 26 m'n Inventor(s) Horace L. Smith, Jr.

It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Qolumn 1, line 6 of the specification, change "continuus" to --eontinuous--.

line 6, change "satisfacgory" to --satisfactory--.

Column 4, line 43, "proudct" should read product fiolumn 5, line 2, change "maintinaing" to --maintaining-.

line 2l, change "structrual" to --structural--.

Gofliumm 3 E 7 line 3, change "188" to --190-.

7 lines 12, and 13, change "absorbing" to --absorbing-.

Column 7, lines 25, and 26, change "cndensor" to --condensor--.

Column 8, line 4, change the comma to a semicolon (7) In the drawing "'CONCENTRATED" in the label appended to element 206 in'Figure 1 should be DILUTE as is apparent from the paragraph spanning lines 36-42, column 7 of the patent.

Signed and sealed this 17th day of September 1974,

(SEAL) Attest:

McCOY M. GIBSON JR, C.. MARSHALL DANN Attesting Officer Commissioner of Patents FORM F o-I050 (10'69) USCOMM DC eoanhpeg fr u s. novermuzm PRINTING orncz; was o-ass-saa 

1. Freeze drying apparatus comprising: a vacuum vessel; means dividing said vacuum vessel into a freezing compartment and a drying compartment; means for evacuating said vessel; means for conveying a product to be dried from said freezing compartment into and through said drying compartment; means for introducing a liquid or semiliquid product to be dried into the freezing compartment and onto the conveying means in a layer, means for cooling said conveying means to freeze the product thereto; means for keeping the pressure in the freezing compartment at a selected level which is sufficiently high to prevent volatiles from evolving from the product until it is frozen to the conveying means and to prevent the product from freezing until it is on the conveying means; means for heating the product as it is conveyed through the drying compartment to evolve water therefrom; and means for removing the dried product from the vessel.
 2. The freeze drying apparatus of claim 1, wherein the means for providing water vapor in said freezing compartment comprises a receptacle in said freezing compartment, means for supplying water to said receptacle, and means for maintaining the water in said receptacle at a temperature sufficiently high that said water will volatilize at the pressure in the freezing compartment.
 3. The freeze drying appartus of claim 1, wherein the means for cooling the product conveying means comprises means for effecting the flow of a refrigerant into heat exchange relationship with said conveyor means.
 4. Freeze drying apparatus according to claim 1, wherein the product conveying means has a material bearing leg and wherein the means for cooling said conveying means comprises a reservoir beneath said material bearing leg and means for effecting a flow of said coolant into said reservoir and out the upper end thereof and into heat transfer relationship with said material bearing leg.
 5. The freeze drying apparatus of claim 1, wherein the means for maintaining the pressure in the freezing compartment at the selected level is means for maintaining an atmosphere of water vapor in said freezing compartment.
 6. The freeze drying apparatus of claim 5, wherein there is a partition in said vacuum vessel dividing it into said freezing and drying compartments, there being apertures in said partition through which said conveyor can pass and said apparatus further including means for removing from said vacuum vessel moisture evolved from the product in the drying compartment and water vapor flowing from the freezing compartment into the drying compartment through the apertures in the partition.
 7. The method of freeze drying a product, comprising the steps of: introducing the product to be dried into a zone maintained at a pressure which is below atmospheric but is sufficiently high to keep volatile constituents of said product from vaporizing; spreading the product into a layer; freezing the product in said layer; reducing the pressure on the frozen product to a pressure below 4.6 mm of mercury; and heating the product to effect the sublimation of water from the product and thereby reduce its moisture content.
 8. The method of claim 7, wherein said product is spread into a layer which is not more than one-sixteenth inch thick.
 9. The method of claim 7, wherein the selected pressure in said zone is obtained by maintaining therein an atmosphere of evaporated water. 